I didn't really notice this about myself until a year ago (it's actually rather frustrating too). I'll try and explain:
Essentially, in one on one situation, each person is focused on the other. At all times more or less. The minimum is they are sort of aware of what they are doing at any given moment. In one on one situations, I feel really pressured to be "on" the entire time. Keep the conversation flowing, making sure I say/do the right thing, making sure they don't seem uncomfortable. Because they are focused on me, I can't drift away, focus on something else, or become inattentive. It's like I am under the microscope and I can't make any mistake. Granted, I have some friends where we can just be in each others presence and be ok, but it takes me a very, very long time (years) before I get to that point. Further, the vast majority of the people I meet I will never get to that point with.
In a group setting, I don't need to be "on" all the time. There are other people around to hold the torch. I can go from speaking and communcating, to sitting and observing. I don't need to worry about my apperance much anymore, and I can easily flit from one person to another if they conversation "dies" and begins to appear out of place. There's more going on and way more material to work with. If I tire, I can take out my phone, or stare off into space, and just not do anything. Everyone else will be able to carry on just fine, and I can jump back into it when I am ready. The entire dynamic feels far less confining. Additionally, I rather like having an "audience". Partly because it's just more fun, and partly because it allows me to be louder, more "dramatic", and informative to everyone. In a one on one situation this would just come across as really weird and forceful (and generally just not socially acceptable) so I can be more of my "natural" self with more people around as the situation allows makes it implicitly more acceptable. Thus I feel far less constrained in my behavior. I could act like this in a one on one setting, but that requires profound comfort with that person to come out of my shell completely like that. There is really only 1 person (technically 2) that I feel that comfortable with.
This is sort of a problem though. It's not easy to form groups like this. It takes a lot of time, a lot of effort, and a lot of luck too. Everyones got to have a good level of comfort with each other. Not super high, but enough. You also can't easily "call up" an entire group as you could with an individual person. So very often I feel this level of social starvation because one on one is not often fulfilling to me. If anything it makes me feel worse, because the entire time I felt strained. Though with close friends it doesn't happen as much. Even then I prefer more people with close friends. I can pull them away for some one on one time in groups anyway. Having other people around adds comfort too because again, they can be bounced off, it soothes the social atmnosphere. It's also hard because socially I am very picky. If one person is in the group that I don't want there it can stink up the whole place for me.
This is a big reason why I feel a persistent loneleness because I just can't get comfortable completely one on one. I want to so so so so bad but I don't know how to do it right. I love group settings, they are very fullfilling, but at my core what I really want is a 1 on 1 bond.
Does that make sense? I've never written this out before, and having done so it actually appears weird even to *me* to realize I am this way lol.